1. Field of the Invention
This invention relates generally to the field of mechanical techniques for altering the audio frequency response of a microphone and more particularly to a microphone mounting arrangement which utilizes a resonant chamber, bounded by a speaker and a resilient diaphragm, to alter a microphone's audio frequency response in a radio transceiver.
2. Background of the Invention
A significant problem encountered in the development of two-way communication systems is that of improving the intelligibility of speech when the listener or transmitter is in a high noise level environment. The listener might encounter difficulty understanding a transmitted speech message in the presence of noises such as automobile traffic, factory and industrial machinery, farm, and military machinery noises. The audio in such a communication system is usually band limited between approximately 300 Hz and 3 KHz, and fortunately most such noises are predominently in the lower end of this frequency range, whereas the information necessary for good speech intelligibility is predominantly in the upper portion of this frequency range. It is well kwown in the art that this is due primarily to the greater significance of the second formant relative to the first formant for the intelligibility of speech. It has been well established that high pass filtering by electrical circuits of the transmitted audio signals in a two-way communication system is an effective method of improving the intelligibility in a high noise environment.
The intelligibility improvement attainable by electrical high pass filtering of the transmitted audio in the 300 to 3000 Hz range in order to emphasize the higher frequencies relative to the lower and middle frequencies is well documented in papers such as "The Enhancement of Speech Intelligibility in High Noise Levels by High Pass Filtering Followed by Rapid Amplitude Compression" by Russell J. Niederjohn and James H. Grotelueschen appearing in Volume ASSP--24, Number 4, August 1976 of the IEEE Transaction on Acoustics, Speech and Signal Processing. FIG. 1 of that article clearly shows that, for one example, in a 90 DB sound pressure level environment speech intelligibility improved from approximately 38% recognition to as much as approximately 82% recognition at a 0 DB signal to noise ratio as a result of adding electrical high pass filtering to the transmitted audio signal.